The invention relates to a cellular cordless telecommunications system.
The cordless telecommunications system with the best performance of the moment, based on the DECT Standard [Digital Enhanced (previously: European) Cordless Telecommunications; cf. Nachrichtentechnik Elektronik [Electronic Information Technology] 42 (January/February 1992), No. 1, Berlin; U. Pilger: .sup.33 "Struktur des DECT-Standards" [Structure of the DECT Standard]; pages 23 to 29 in conjunction with the ETSI Publication ETS 300175-1 . . . 9, October 1992; Siemens Components 31 (1993), No. 6; S. Althammer and D. Bruckmann: "Hochoptimierte IC's fur DECT-Schnurlostelefone [Highly optimized ICs for DECT cordless telephones]", pages 215 to 218; telecom report 16 (1993), No. 1, J. H. Koch: "Digitaler Komfort fur schnurlose Telekommunikation--DECT-Standard eroffnet neue Nutzungssgebiete [Digital convenience for cordless telecommunications--DECT Standard opens up new fields of use]", pages 26 and 27), transmits the radio messages, inter alia, using the TDMA method (Time Division Multiple Access).
However, the present invention is not limited to the TDMA transmission method. The invention thus also extends to those systems which are described in the document TIB-RO9067(5)-P. Bauer-Trocheris: "UMTS-Integrator fur die mobile Kommunikation--ein Ausblick auf die Mobilfunklandschaft nach dem Jahr 2000 [UMTS integrator for mobile communications--a summary of the mobile radio situation after the year 2000]", Report on the Convention "European Mobile Radio, 5th annual meeting of the German and European mobile radio section, FIBA Congress, Munich, Feb. 24 to 26, 1993".
The DECT-specific cordless telephone system is a digital system which, according to FIG. 1 (cf. tec 2/93--Technical Magazine from Ascom "Wege zur universellen mobilen Telekommunikation [Ways for universal mobile telephone communication]", pages 35 to 42) can be used in the private area (for example house, apartment, garden etc.), in the small public area (for example companies, office building etc.) and as a telepoint application.
The basic structure of the cordless telephone system comprises a base station BS (FP=Fixed Part with FT=Fixed Termination and a mobile unit MT (PP=Portable Part with PT=Portable Termination with the capability for telecommunications with the base station BS. This basic structure can be extended in accordance with the DECT Standard in such a manner that up to twelve such mobile units MT are assigned to a single base station BS.
FIG. 2 shows such a cordless telephone system STS, in which a maximum of 12 links using the TDMA/FDMA/TDD method (Time Division Multiple Access/Frequency Division Multiple Access/Time Division Duplex) are set up on one DECT base station BS in parallel to DECT mobile units MT1 . . . MT12 via a DECT radio interface designed for the frequency band between 1.88 and 1.90 GHz. The FIG. 12 results from a number "k" of time slots or telecommunications channels (k=12) available for duplex operation of a DECT system. The links may in this case be internal and/or external. In the case of an internal link, two mobile units registered at the base station BS, for example the mobile unit MT2 and the mobile unit MT3, can communicate with one another. For setting up an external link, the base station BS is connected to a telecommunications network TKN, for example in wire-based form via a telecommunications access unit TAE or a private branch exchange NStA is connected to a wire-based telecommunications network or, in accordance with WO 95/05040, in wire-free form as a repeater station to a higher level telecommunications network. In the case of the external link, it is possible to communicate with a mobile unit, for example with the mobile unit MT1, via the base station BS, and for the telecommunications access unit TAE and a private branch exchange NStA to communicate with a subscriber in the telecommunications network TKN. If, as in the case of the Gigaset 951 (Siemens cordless telephone, cf. telecom report 16, (1993), issue 1, pages 26 and 27--the base station BS has only one connection to the telecommunications access unit TAE or the private branch exchange NStA, then only one external link can be set up. If--as in the case of the Gigaset 952 (Siemens cordless telephone, cf. telecom report 16, (1993), issue 1, pages 26 and 27--the base station BS has two connections to the telecommunications network TKN, then, in addition to the external link to the mobile unit MT1, a further external link is possible from a wire-based telecommunications terminal TKE connected to the base station BS. In this case, it is in principle also feasible for a second mobile unit, for example the mobile unit MT12, to use the second connection for an external link, instead of the telecommunications terminal TKE. While the mobile units MT1 . . . MT12 are operated using a battery or a rechargeable battery, base station BS, which is designed as a small cordless switching system, is connected via a mains connecting unit NAG to a power network SPN.
The cordless telephone system according to FIG. 2 is preferably used in the private area, according to FIG. 1.
In the small public area--in accordance with FIG. 1--a plurality of such cordless telephone systems according to FIG. 2 can be operated in a cellular system on a private branch exchange (Private Automatic Branch Exchange), the private branch exchange PABX controlling a plurality of base stations A-BS, B-BS, C-BS and, if required, supporting a handover from one base station to another. FIG. 1 also shows six mobile units MT.sub.a . . . MT.sub.f which are assigned to the three base stations A-BS, B-BS, C-BS. This results in a cellular cordless telephone system in which the telecommunications link as a rule passes via that base station A-BS, B-BS, C-BS with which the mobile unit MT.sub.a . . . MT.sub.f has the best radio contact.
Based on the document "Nachrichtentechnik Elektronik [Electronic Information Technology]"42 (1992) January/February, No. 1, Berlin, DE; U. Pilger: "Struktur des DECT-Standards [Structure of the DECT Standard]", pages 23 to 29 in conjunction with the ETS Publication ETS 300175-1 . . . 9, October 1992, FIG. 3 shows the TDMA structure of the DECT system STS. In terms of multiple access methods, the DECT system is a hybrid system in which radio messages can be transmitted on ten frequencies in the frequency band between 1.88 and 1.90 GHz using the FDMA principle and, according to FIG. 3, in a predetermined time sequence using the TDMA principle, from the base station BS to the mobile unit MT and from the mobile unit MT to the base station BS (duplex operation). The time sequence is in this case governed by a multiple time frame MZR, which occurs every 16 ms and which has 16 time frames ZR, each having a time duration of 10 ms. Information is transmitted separately in these time frames ZR to the base station BS and to the mobile unit MT, this information relating to a C, M, N, P and Q channel defined in the DECT Standard. If information for a number of these channels is transmitted in one time frame ZR, then the transmission takes place based on a priority list where M&gt;C&gt;N and P&gt;N. Each of the 16 time frames ZR in the multiple time frame MZR is in turn split into 24 time slots ZS each having a time duration of 417 .mu.s, of which 12 time slots ZS (time slots 0 . . . 11) are reserved for the transmission direction "base station BS.fwdarw.mobile unit MT" and a further 12 time slots ZS (time slots 12 . . . 23) are reserved for the transmission direction "mobile unit MT.fwdarw.base station BS". Information having a bit length of 480 bits is transmitted in each of these time slots ZS, in accordance with the DECT Standard. Of these 480 bits, 32 bits are transmitted as synchronization information in a SYNC field, and 388 bits as wanted information in a D field. The remaining 60 bits are transmitted as additional information in a Z field and as guard information in a "Guard Time" field. The 388 bits in the D field transmitted as wanted information are in turn split into a 64-bit long A field, a 320-bit long B field and a 4-bit long "X-CRC" word. The 64-bit long A field is composed of an 8-bit long data header, a 40-bit long data set with data for the C, Q, M, N and P channels and a 16-bit long "A-CRC" word.
Based on the document Siemens Components 31 (1993), Issue 6, pages 215 to 218; S. Althammer, D. Bruckmann: "Hochoptimierte IC's fur DECT-Schnurlostelefone [Highly optimized ICs for DECT cordless telephones]", FIG. 4 shows the basic circuit layout of the base station BS and of the mobile unit MT. According to this, the base station BS and the mobile unit MT have a radio section FKT with an antenna ANT which is associated with transmission and reception of radio signals, a signal processing device SVE and a central controller ZST, these items being linked to one another in the manner illustrated. The radio section FKT essentially contains known devices such as the transmitter SE, receiver EM and synthesizer SYN as well as a field strength measuring device RSSI (Radio Signal Strength Indicator, which are linked with one another in a known manner. The signal processing device SVE contains, inter alia, an encoder/decoder device CODEC. The central controller ZST has a microprocessor .mu.P both for the base station BS and for the mobile unit MT, with a program module PGM designed in accordance with the OSI/ISO (International Organization of Standardization/Open Standardization/Open System Interconnection) layer model (cf. Information leaflets--Deutsche Telekom year 48, 2/1995, pages 102 to 11 ETSI Publication ETS 300175-1 . . . 9, October 1992), a signal control section SST and a digital signal processor DSP, which are linked to one another in the manner illustrated. Of the layers defined in the layer model only the first four layers, which are absolutely essential for the base station BS and the mobile unit MT are illustrated. The signal control section SST in the base station BS is designed as a Time Switch Controller TSC, and that in the mobile unit MT is designed as a Burst Mode Controller BMC. The major difference between the two signal control sections TSC, BMC is that the base-station-specific signal control section TSC also carries out switching functions (switch functions) in comparison with the mobile-unit-specific signal control section BMC. The signal control sections TSC, BMC each contain a counting device ZE with a bit, time slot and time frame counter.
The basic method of operation of the circuit units mentioned above is described, for example, in the document cited above, Siemens Components 31 (1993), Issue 6, pages 215 to 218.
The described circuit layout according to FIG. 4 in the base station BS and the mobile unit MT is supplemented by additional functional units in accordance with their function in the DECT system according to FIG. 2.
The base station BS is connected to the telecommunications network TKN via the signal processing device SVE and the telecommunications access unit TAE or the private branch exchange PABX. As an option, the base station BS may also have an operator interface (functional units shown by dashed lines in FIG. 4), which comprises, for example, an input device EE designed as a keypad, an indicating device AE designed as a display, a speech/listening device SHE designed as a handset with microphone MIF and an earpiece HK, as well as a ringer bell TRK.
The mobile unit MT has the operator interface (which is possible as an option for the base station BS) with the control elements which are part of this operator interface and are described above.
Like the base station BS in the private area, the private branch exchange PABX according to FIG. 1 is connected to the wire-based public telephone network PSTN (Public Switched Telephone Network). In this way, anyone can become his own network operator by obtaining a cordless telecommunications system comprising the private branch exchange PABX and the cordless telephones A-BS, B-BS, C-BS, MT.sub.a . . . MT.sub.f connected to it. EP-0466736 B1 discloses a cordless telephone system having a private branch exchange in which the private branch exchange performance features are used to set up a cellular cordless telecommunications system.
In order to allow network coordination of cordless telephones according to FIG. 1 to be dispensed with, the Dynamic Channel Allocation method (DCA method) is provided in accordance with the DECT Standard. If, for example, a DECT link is being set up, a search is carried out for that frequency and that time window having the least interference. The level (intensity) of the interference depends primarily on whether
(a) a conversation is already being carried out on another base station, or PA1 (b) as a result of movement, a mobile unit comes into visual contact with a base station that was previously screened.
Any increase in interference resulting from this can be countered using the TDMA transmission method on which the DECT cordless telephone system is based. According to the TDMA method, only one time slot is used for the actual transmission; the other eleven time slots can be used for measurements. In consequence, it is possible to determine an alternative frequency/time slot pair, to which the link can be switched. This is done in the course of adaptive channel application in accordance with the DECT Standard (cf. Nachrichtentechnik Elektronik [Electronic Information Technology] 42 (January/February 1992), No. 1, Berlin; U. Pilger: "Struktur des DECT-Standards [Structure of the DECT Standard]"; page 28, paragraph 3.2.6) by means of a "Connection Handover" (Intra-Cell Handover).
In addition to this "Intra-Cell Handover", the "Inter-Cell Handover" or the seamless handover may be mentioned, which is likewise possible in the course of DECT-specific adaptive canal allocation.
In order now to address in particular the "Inter-Cell Handover" problem which occurs regularly in cellular wire-free telecommunications systems, the mobile radio receiver (mobile unit) provided for such cellular radio telecommunications systems, must be able to change the base station, subject to a cell change within the cellular radio system, at any time during an active telecommunications link to a (quasi-) stationary radio transmitter (base station), (setting up a telecommunications link to another base station) while, at the same time, passing on the already existing active telecommunications link without any interruption (seamlessly) to the other base station (seamless handover).
According to the document Nachrichtentechnik Electronik [Electronic Information Technology]" 42 (January/February 1992) No. 1, Berlin; U. Pilger: "Struktur des DECT-Standards [Structure of the DECT Standard]"; page 28, paragraph 3.2.6, the DECT Standard provides for this purpose that the mobile unit autonomously sets up a second telecommunications link, in parallel with the existing link, in the event of any deterioration in the transmission quality of the existing telecommunications link, on the basis of indicators of the transmission quality (for example signal field strength, CRC values, etc.). In the case of this "Inter-Cell Handover" procedure, the fact that DECT mobile units are continuously informed of the status of the channels available in the current environment in the course of the dynamic, decentralized channel allocation process (DCA method), is used in such a manner that the second link is set up on the basis of the entry in a channel list.
An interruption-free handover is possible using the above procedure only if the mobile unit is located in a cellular radio system with synchronized base stations. In such a synchronous cellular radio system, the mobile unit can then, in addition to the already existing telecommunications link to a base station (original base station), set up at least one further link to another base station in another radio cell without losing the synchronism with the original base station in the process. Such a synchronous cellular radio system can, however, be implemented only with considerable system complexity (cable or radio synchronization).
U.S. Pat. No. 5,388,102 discloses an arrangement for synchronization of a large number of base stations in a wire-free telecommunications system, these base stations being connected to a switching center via ISDN basic accesses. With regard to the handover problems in wire-free telecommunications systems, it is expedient if the base stations arranged in the telecommunications system are synchronized to one another. In order to synchronize the base stations, a synchronization signal (synchronization message) is therefore transmitted from the switching center to the individual base stations in the course of a communication protocol which takes place between the switching center and the base stations. Where ISDN basic access that is used in the known arrangement is used as the transmission medium between the switching center and the base stations, the S bit or M bit in the ISDN data structure is used, for example, as the carrier for the synchronization signal.
European Reference 0 529 359 A2 discloses a private branch exchange having an integrated mobile unit, by means of which cordless terminals which can be operated via base stations can be controlled. For this purpose, the private branch exchange has freely configurable interface modules. A central control unit is provided for controlling all the switching and performance feature control tasks, and a lower-level control unit is provided for controlling the setting up and clearing of links between the cordless terminals. The control units and the interface modules are connected to one another via a BUS system. With the known private branch exchange, it is furthermore possible to access and cordless terminal anywhere within the radio range of the base stations and to pass on the existing link, controlled by the lower-level control unit, when the radio area of the cordless terminal changes from one base station to another base station.